1. Field of the Invention
The invention is directed to methods and compositions useful for enhancing adhesion and protecting electrode layers from corrosion, rust or oxidation.
2. Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. An electrophoretic fluid composed of a colored solvent with charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side or the other causing either the color of the pigment particles or the color of the solvent being seen from the viewing side.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of particles, such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. To complete the module assembly, the microcapsules may be coated directly on an electrode and subsequently laminated with a second electrode layer. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology was disclosed in a co-pending application, U.S. Ser. No. 09/518,488 filed on Mar. 3, 2000 (corresponding to WO 01/67170), the content of which is incorporated herein by reference in its entirety. The improved EPD cells may be prepared, for example, by microembossing a layer of a thermoplastic or thermoset precursor composition coated on a first substrate layer to form microcups of well-defined shape, size and aspect ratio. The microcups are then filled with an electrophoretic fluid and top-sealed with a sealing layer. A second substrate layer is laminated over the filled and sealed microcups, preferably with an adhesive layer.
Another co-pending application, U.S. Ser. No. 10/351,460 filed on Jan. 24, 2003, discloses finished and semi-finished display panels prepared from the microcup technology. The content of U.S. Ser. No. 10/351,460 is also incorporated herein by reference in its entirety.
For an electrophoretic display having an up/down or dual switching mode, a display cell layer is sandwiched between two electrode layers. For an electrophoretic display having an in-plane switching mode, a display cell layer is sandwiched between an insulating layer and an electrode layer.
The electrode layer may be a thin film transistor (TFT) back plane or a printed circuit board (PCB), particularly a flexible PCB. Unfortunately, it is a well-known problem that the electrode (such as copper) traces and some solders or conductor coatings or inks typically used in PCBs or a TFT back plane are susceptible to corrosion, rust or oxidation. As a result, the surface color of the electrode layer has a tendency to turn from a bright and shiny color to a reddish color with an uneven pattern, after prolonged exposure to air and/or humidity. The oxidation of the electrode traces, solders and conductive coatings or inks not only causes a color change and forms a pattern of an uneven color, but also causes a reduction in conductivity and an adverse effect on adhesion of the electrode layer to other components in the display.
This problem may be solved by protecting the electrode layer surface from corrosion, rust or oxidation by, for example, coating a protective coating such as a solder mask or an inert conductive material, such as gold, over the surface of the electrode layer soon after the electroplating, lithographic or printing steps. However, the solder mask is a thick, rigid and thermally resistant thermoset coating and it is designed to be non-tacky and therefore is not suitable as an adhesive. The gold coating, on the other hand, is expensive and the adhesion of gold to conventional adhesives is not suitable for many applications.
The need of protecting the electrode surface during handling, storage and shipping without the above-mentioned detrimental effect on the EPD performance presents a challenge in logistic or inventory control for mass production of EPD modules.